Carburetters for internal combustion engines

ABSTRACT

The piston assembly of a carburetter of the controllable jet automatic variable-choke type includes a piston rod having a bore extending through it. Displacement of the rod is damped by means of a plunger mounted in the bore which in use contains a liquid. To vary the damping action of the plunger in dependence on the relative axial position of plunger and rod the bore is shaped to vary the plunger-bore clearance.

United States Patent [191 Swatman et al.

[ CARBURETTERS FOR INTERNAL COMBUSTION ENGINES [75] Inventors: Peter P. Swatman, Solihull; John Edgar Tullett, Knowle, both of England [73] Assignee: British Leyland (Austin-Morris) Limited, Birmingham, England [11] 3,819,159 June 25, 1974 4/1971 Great Britain 261/44 R 4/1967 Great Britain 261/50 A Primary Examiner-Tim R. Miles Attorney, Agent, or Firm-Brisebois & Kruger [57] ABSTRACT The piston assembly of a carburetter of the controllable jet automatic variable-choke type includes a piston rod having a bore extending through it. Displacement of the rod is damped by means of a plunger mounted in the bore which in use contains a liquid. To vary the damping action of the plunger in dependence on the relative axial position of plunger and rod the bore is shaped to vary the plunger-bore clearance.

5 Claims, 2 Drawing Figures CARBURETTERS FOR INTERNAL COMBUSTION ENGINES This invention relates to a carburetter for an internal combustion engine. In particular it is concerned with a carburetter of the controllable jet automatic variablechoke type in which a fuel metering jet, which constitutes a fuel-discharge orifice, is controlled by a contoured metering needle in dependence upon the instantaneous position of a suction operated member. The suction operated member further functions to control the cross-sectional area of a choke in a main air passage in the carburetter. The suction operated member usually comprises a piston assembly made up of a suction disc, slidably disposed in a suction chamber, and a piston rod mounted on the suction disc and slidable in a bearing afforded by a hollow cylindrical guide disposed centrally in the suction chamber.

The piston rod has a parallel walled bore extending through it and has disposed within it a plunger attached by a rod to a cap secured to the upper end of the suction chamber. The piston rod bore is closed at its lower end and is filled with oil. The plunger is so valved that relative upward movement past the plunger of the piston rod (caused by movement of the suction disc) causes compression of the oil filling beneath the plunger and so damping of the movement of piston rod and suction disc. The plunger valving allows the piston rod to drop freely past the plunger when the pressure differential across the suction disc generates downward movement. Hereafter such a carburetter is referred to as being of the type described.

1 According to the present invention a carburetter of the controllable jet automatic variable-choke type in which movement of a suction operated member is used to control the size of a choke in an air passage of the carburetter; the suction operated member comprising a piston assembly made up of a suction disc slidably disposed in the suction chamber and a piston rod mounted on, and for movement with, the suction disc, between a first position when the choke size is least and a second position in which the choke size is greatest; the piston rod having a bore extending through it; the suction chamber having secured in it a plunger so ex tending into the bore of the piston rod as to allow movement of the piston rod past the plunger is characterised in that the bore of the piston rod is so shaped that clearance between bore and plunger varies as the piston rod passes from the first to the second position.

In a first preferred form of the invention the bore is so adapted that clearance is least with the piston rod at, and in the vicinity of, the first position and is greatest with the piston rod at, and in the vicinity of, the second position.

In a second preferred form of the invention the bore is tapered over at least a part of its length.

Carburetters embodying the invention will now be described, solely by way of example, with reference to the accompanying drawings of which FIGS. 1 and 2 show cross sectional views of part of two S.U. carburetters.

Both Figures show a carburetter piston assembly 11 including a suction disc 12. Lower part 10 of the assembly is used to control the effective cross-sectional area of a choke in the main air passage of the carburetter. The disc 12 is slidable within a suction chamber 14 whose internally threaded tubular upper end 15 is closed by a screwed cap 16. Extending downwardly from, and coaxial with, the upper end 15 is a hollow cylindrical guide 17. The screwed cap 16 is provided with a downwardly extending rod 18 whose end remote from the cap 16 retains a plunger 19.

In FIG. 1 the suction disc 12 has secured to it a piston rod 13 whose upper region is slidably disposed within the hollow cylindrical guide 17. The piston rod 13 is hollow and is closed at its lower end by plug 20 which serves to define one end of a chamber 21 within the rod 13. The chamber 21 is filled, with oil of appropriate viscosity for the damping action required. Walls of the chamber 21 are defined by a lower section 22, middle section 23 and upper section 24. Both lower section 22 and upper section 24 are defined by parallel sided but differing sized bores. However the middle section 23 has a relatively short tapered wall linking the upper and lower sections. Plunger 19 is disposed coaxially within the chamber 21.

In FIG. 2 the suction disc 12 has secured to it a piston rod 31 whose upper region is slidably disposed within the hollow cylindrical guide 17. The piston rod 31 is hollow and is closed at its lower end by a plug 32 which serves to define one end of a chamber 33 within the rod 31. The chamber 21 is filled with oil of appropriate viscosity for the damping action required. Walls of the chamber 33 are defined by a tapered lower section 34 and a parallel sided upper section 35. Plunger 19 is dis posed coaxially with chamber 33.

In the case of both FIG. 1 and FIG. 2 the relative position of the parts is that in which the lower part 10 of piston assembly 11 provides a minimum cross-sectional area of a choke in the main air passage. On creating a suitable differential across suction disc 12 the assembly 11 rises so increasing the cross-sectional area of the choke.

In FIG. 1 upwards movement of piston rod 13 consequent upon the assembly 11 rising causes the upper section 24 of chamber 21 to move: past the plungers 19 into the middle section 23 with a consequent reduction in the damping effect in view of the increasing clearance between plunger 19 and the outwardly tapering middle section 23. Continuing upwards movement of the piston rod 13 causes the lower section 22 to be raised past the plunger 19 in which section the damping action of the plunger 19 on the moving piston rod 13 is minimal. Downward movement of the piston assembly 11 results in the piston rod 13 dropping past the plunger 19 so resulting in an increasing damping action on the rod motion by the plunger 19 with the reducing clearance between plunger and the wall sections of chamber 21. The axially short length of middle section 23 compared with sections 24 and 22 causes a rapid change in damping action to occur on movement of the rod past the plunger.

In FIG. 2 upward movement of the piston rod 31 causes the upper section 25 of chamber 33 to move past the plunger 19 into the gently tapering lower section 34 with a consequently progressive reduction in damping effect in view of the increasing clearance be tween plunger 19 and the lower section 34. Downward movement of the piston assembly 11 results in the reverse of the above damping effect.

It will be apparent on a comparison of FIGS. 1 and 2 that the relatively sharp change in cross-sectional area of midle section 23 (FIG. 1) will produce a more 3 rapid change in damping action for a given piston rod speed than the more gently tapered lower section 34 (FIG. 2).

The carburetters described are intended for use on internal combustion engines for road vehicles. Carburetters according to the embodiments will enable a more apt response to be obtained in the event of rapidly varying engine speed.

We claim:

1. A carburetter of the controllable jet automatic variable-choke type in which movement of a suction operated member is used to control the size of a choke in an air passage of the carburetter; the suction operated member comprising a piston assembly made up of a suction disc slidably disposed in a suction chamber and a piston rod mounted on, and for movement with, the suction disc between a first position when the choke size is least and a second position in which the choke size is greatest; the piston rod having a bore extending through it; the suction chamber having secured in it a plunger so extending into the bore of the piston rod as to allow movement of the piston rod past the plunger; characterised in that the bore of the piston rod is so shaped that clearance between bore and plunger varies as the piston rod passes from the first to the second position.

2. A carburetter as claimed in claim 1 wherein the bore is so adapted that clearance is least with the piston rod at, and in the vicinity of, the first position and is greatest with the piston rod at, and in the vicinity of, the second position.

3. A carburetter as claimed in claim 2 wherein the bore is tapered over at least a part of its length.

4. An internal combustion engine equipped with a carburetter as claimed in claim 1.

5. A vehicle equipped with an internal combustion engine as claimed in claim 4.

. UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Pateni- No. i I 3 819, 159 i I Dat zed June 25J 197 i Ihventofls) PETER P. SWATMAN & JOHN EDGAR TULLETT It'is certified that error appears in the above-identified patvnt and that said Letters Patent are hereby corrected as shown below:

[30] Foreign, Application Priority Data March lj, 1972 Great Britain 11926/72 Signed and sealed this 17th day of September 1974 (S EA L) Attest:

MeCOY M. GIBSON JR. c. MARSHALL DANN Atte'sting Officer Commissioner of Patents FORM PO-105O HO-69) USCONIM'DC 6O376-w69 U S, GOVERNME I PRINTING OFFICE: IQQ 0-356-33 l 

1. A carburetter of the controllable jet automatic variablechoke type in which movement of a suction operated member is used to control the size of a choke in an air passage of the carburetter; the suction operated member comprising a piston assembly made up of a suction disc slidably disposed in a suction chamber and a piston rod mounted on, and for movement with, the suction disc between a first position - when the choke size is least - and a second position - in which the choke size is greatest; the piston rod having a bore extending through it; the suction chamber having secured in it a plunger so extending into the bore of the piston rod as to allow movement of the piston rod past the plunger; characterised in that the bore of the piston rod is so shaped that clearance between bore and plunger varies as the piston rod passes from the first to the second position.
 2. A carburetter as claimed in claim 1 wherein the bore is so adapted that clearance is least with the piston rod at, and in the vicinity of, the first position and is greatest with the piston rod at, and in the vicinity of, the second position.
 3. A carburetter as claimed in claim 2 wherein the bore is tapered over at least a part of its length.
 4. An internal combustion engine equipped with a carburetter as claimed in claim
 1. 5. A vehicle equipped with an internal combustion engine as claimed in claim
 4. 